In a transmission-type electron microscope, an electron beam transmitted through a filmy specimen is imaged by an imaging lens system to obtain an electron optical image of the specimen. Generally, this electron optical image is visualized with a photosensitive material. It is the common practice to use a photographic film to which emulsion has been applied as the photosensitive material.
Referring to FIG. 1, the structure of a conventional electron microscope using photographic films is shown. The microscope includes a microscopic column 1 in which an electron optical system for obtaining an electron optical image is housed. A camera chamber 2 is provided to visualize the electron optical image. An electron gun 3 for producing an electron beam 4 is mounted in the microscopic column 1. The beam 4 is converged by condenser lenses 5 and impinges upon a filmy specimen 6. An objective lens 7 and magnifying lenses 8 constitute an imaging lens system which produces a magnified electron optical image of the specimen from the electron beam transmitted through the specimen 6.
The electron optical image is made visible by a micrograph-taking section including a shutter, a fluorescent screen 25, and the camera chamber 2. The fluorescent screen 25 consists of an aluminum plate 25a and a fluorescent substance layer 25b coated on the one side surface of the aluminum plate 25a. The visible image on the fluorescent screen 25 is observed through the observation window 26. The shutter 9 is disposed near the bottom of the microscopic column 1. A feeding magazine 12 holding unexposed films, a receiving magazine 13 holding exposed films 15, and a carrying mechanism 10 are mounted in the camera chamber 2. The mechanism 10 carries a film 11 from the feeding magazine 12 into a location where the film is exposed to the electron optical image. Then, the mechanism 10 moves the film from the exposure position into the receiving magazine 13. A mask 16 is disposed above the film 11 located in the exposure position to determine the field of view. During exposure operation, the fluorescent screen 25 is vertically inclined upwardly toward the axis of the microscope as shown by 25', so that the electron optical image is projected upon the film 11 through the magnifying lenses 8. The electron optical image is projected upon the film supplied from the feeding magazine 12 for a period determined by the shutter 9. The exposed film is then received in the receiving magazine 13.
In the electron microscope shown in FIG. 1, the electron beam 4 impinges upon the shutter 9 immediately before and after the exposure operation, thus producing x-rays. During the exposure operation, the electron beam 4 hits the mask 16, also producing x-rays. Because the conventional electron microscope uses a photographic film as a recording medium, and because the photographic film is insensitive to x-rays, it is possible to expose the film only to the electron optical image. Hence, the produced x-rays present no problems.
In recent years, a new electron microscope system making use of a two-dimensional sensor to store the energy of an electron beam has been proposed. In particular, the sensor consists of a storage-type phosphor sheet or the like. An electron optical images is recorded on the sensor. Then, the sensor is illuminated with light or heated to release the stored energy as light. The emitted light is photoelectrically detected to produce an image signal. A transmission electron micrograph of a specimen is reconstructed from the image signal (see European patent application No. 0168838). Generally, two-dimensional sensors of this kind are sensitive to x-rays and, therefore, they are exposed to x-rays produced from the shutter 9 and the mask 16 as well as to extraneous x-rays. These undesired x-rays are superimposed on the electron optical image. As a result, the obtained electron micrograph is blurred.
When the aforementioned two-dimensional sensor is exposed to an electron beam, it temporarily stores at least a portion of the energy of the beam. When the sensor is subsequently excited externally, at least a portion of the stored energy is released in a detectable form, such as light, electricity, and sound.
FIG. 2 is a cross-sectional view of a stimulable phosphor sheet adapted for the above-described two-dimensional sensor, the sheet being stimulated by stimulating rays. The stimulable phosphor sheet, indicated by numeral 22, comprises a support 23 and a stimulable phosphor layer 24 formed on the base 23. The base 23 can be a sheet of polyethylene or plastic film of about 100 to 200 .mu.m thick, a sheet of aluminum of 0.5 to 1 mm thick, a sheet of glass of 1 to 3 mm thick, or the like. The base 23 may or may not be transparent. Where the base is opaque, the light emitted from the stimulable phosphor sheet is detected from the same side as the stimulating rays impinged. Where the base is transparent, the emitted light can be detected from the opposite side to the stimulating rays impinged on both sides.
The stimulable phosphor used in the stimulable phosphor sheet of the two-dimensional sensor employed in the present invention can be: EQU (Ba.sub.1-x-y, Mg.sub.x, Ca.sub.y)FX:.alpha.Eu.sup.2+ ( 1)
where X is at least one of Cl and Br; and x and y satisfy the conditions 0&lt;x +y.ltoreq.0.6 and xy.noteq.0; and .alpha. satisfies the condition 10.sup.-6 .ltoreq..alpha..ltoreq.5.times.10.sup.-2. This is described in Japanese Patent Unexamined Laid Open No. 12143/1980. EQU LnOX: xA (2)
where Ln is at least one selected from the group consisting of La, Y, Gd and Lu; X is at least one of Cl and Br; A is at least one of Ce and Tb; and x satisfies the condition 0&lt;.times.&lt;0.1. This is described in U.S. Pat. No. 4,236,078.
ti M.sup.II FX.XA: yLn (3)
where M.sup.II is at least one selected from the group consisting of Ba, Ca, Sr, Mg, Zn and Cd; A is at least one selected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, In.sub.2 O.sub.3, SiO.sub.2, TlO.sub.2, ZrO.sub.2, Geo.sub.2, SnO.sub.2 Nb.sub.2 O.sub.5 Ta.sub.2 O.sub.5 and ThO.sub.2 ; Ln is at least one selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, H.sub.O, Nd, Yb, Er, Sm and Gd; X is at least one selected from the group consisting of Cl, Br and I; x satisfies the condition 5.times.10.sup.-5 &lt;.times..ltoreq.0.5; and y satisfies the condition 0&lt;y.ltoreq.0.2. This stimulable phosphor is described in U.S. Pat. No. 4,539,138. EQU BaFX.xNaX':.alpha.Eu.sup.2+ ( 4)
where each of X and X' is at least one of Cl, Br and I; x satisfies the condition 0&lt;.times..ltoreq.2; and satisfies the condition 0&lt;.alpha..ltoreq.0.2. This stimulable phosphor is described in Japanese Patent Unexamined Laid Open No. 56479/1984. EQU M.sup.II X.sub.2..alpha.M.sup.II X'.sub.2 :xEu.sup.2+ ( 5)
where M.sup.II is at least one alkaline-earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; X and X' are different halogens (X.noteq.X'); .alpha. is a numerical value satisfying the condition 0.1.ltoreq..alpha..ltoreq.10.0; x is a value satisfying the condition 0&lt;x.ltoreq.0.2. This stimulable phosphor is described in Japanese patent application No. 84381/1985 (U.S. Ser. No. 834,886), and can contain an additive as described in Japanese patent application No. 166379/1985 (corresponds to European patent application No. 151,494) or 221483/1985 (U.S. Ser. No. 947,631, European patent application No. 159,014).
Other usable storage-type fluorescent substances are described in U.S. Pat. Nos. 3,859,527, 4,236,078, 4,239,968, 4,505,989 and Japanese patent application Nos. 116777/1981, 23673/1982, 23675/1982, 69281/1983, 206678/1983 (U.S. Ser. No. 841,044, European patent application No. 95741), 27980/1984 (European patent application No. 101,030), 47289/1984 (European patent application No. 103,302), 752200/1984 (European patent application No. 107,192) and 101173/1985. Any one of these storage-type fluorescent substances is dispersed in a suitable binder and applied to the support 23 up to a thickness of 1000 microns. If a stimulable phosphor layer can sustain itself, it can form a stimulable phosphor sheet by itself.
When a stimulable phosphor sheet formed in this way is exposed to an electron beam or other radiation, some of the energy of the radiation is stored in the stimulable phosphor. Subsequently, the sheet is exposed to stimulating rays such as visible light. As a result, the fluorescent substance fluoresces according to the stored energy. Instead of the stimulable phosphor stimulated by light, a thermal phosphor can be used which releases the stored energy when heated after it stores radiation energy. In this case, the thermal phosphor is sulfate, such as Na.sub.2 SO.sub.4, MnSO.sub.4, CaSO.sub.4, SrSO.sub.4 or BaSO.sub.4, to which a trace of at least one of the additives Mn, Dy and Tm is added. The thermal phosphor sheet is fabricated in the same manner as the aforementioned stimulable phosphor sheet.
A two-dimensional sensor made of the above-described stimulable phosphor sheet or thermal phosphor sheet is placed in the focal plane of an electron microscope. A transmission electron optical image is stored in this sensor, which is scanned either with stimulating rays such as visible light or with a heat source so that it may emit light. The resulting emitted light is detected photoelectrically. As a result, an electric signal corresponding to the transmission electron optical image can be obtained. The signal derived in this way can be fed to a display unit such as a CRT to make visible the electron optical image, or it can be permanently recorded in the form of hard copy. It is also possible to temporarily store the image signal on a recording medium such as magnetic tape or magnetic disk.
In the above-described electron microscope system, an electron optical image is recorded on a two-dimensional sensor consisting of a storage-type phosphor sheet or the like. Since the sensitivity of the sensor to the electron optical image is high, it is possible to reduce the amount of electron beam impinging upon a specimen under observation. This reduces the damage done to the specimen. Also in this system, the obtained electron micrograph can e quite easily and variously processed. Digitization of image signals and analysis of images and diffraction patterns can be more easily and repaidly performed by applying the aforementioned electric signal to a computer.